1. Technical Field
The present embodiments relate generally to the field of railroad couplers, and more specifically, to the manufacturing of railway couplers and their various parts through the use of no-bake or air-set casting.
2. Related Art
Sand casting is one of the earliest forms of casting. Its popular use is due to its low cost and the simplicity of materials involved. A sand casting or a sand molded casting is a cast part produced through the following process: (1) placing a pattern in sand to create a mold, which incorporates a gating system; (2) removing the pattern; (3) filling the mold cavity with molten metal; (4) allowing the metal to cool; (5) breaking away the sand mold and removing the casting; and (6) finishing the casting, which may include weld repair, grinding, machining, and/or heat treatment operations. This process is now explained in more detail.
In sand casting, the primary piece of equipment is the mold, which contains several components. The mold is divided into two halves—the cope (upper half) and the drag (bottom half), which meet along a parting line. The sand mixture is packed around a master “pattern” forming a mold cavity, which is an impression of the shape being cast. The sand is usually housed in what casters refer to as flasks, which are boxes without a bottom or lid, used to contain the sand. The sand mixture can be tamped down as it is added and/or the final mold assembly is sometimes vibrated to compact the sand and fill any unwanted voids in the mold. The sand can be packed by hand, but machines that use pressure or impact ensure even packing of the sand and require far less time, thus increasing the production rate. The pattern is removed, leaving the mold cavity. Cores are added as required, and the cope is placed on top of the drag.
Cores are additional pieces that form the internal openings, recesses, and passages of the casting. Cores are typically comprised of sand so that they can be shaken out of the casting, rather than requiring the necessary geometry to slide out. As a result, sand cores allow for the creation of many complex internal features. Each core is positioned in the mold before the molten metal is poured. Recesses in the pattern called core prints anchor each core in place. The core may still shift, however, due to poor fit up between core and core prints, the flow of the metal around the core, or due to buoyancy in the molten metal.
Small metal pieces called chaplets are fastened between the cores and the cavity surface to provide further support for the cores. Chaplets are small metal pieces that are fastened between the core and the cavity surface. Chaplets consist of a metal with a higher melting temperature than that of the metal being cast in order to maintain their structure to support the core. After solidification, the chaplets are cast inside the casting and the excess material of the chaplets that protrudes is cut off.
In addition to the external and internal features of the casting, other features must be incorporated into the mold to accommodate the flow of molten metal. The molten metal is poured into a pouring basin, which is a large depression in the top of the sand mold. The molten metal funnels out of the bottom of this basin and down the main channel, called the sprue. The sprue connects to a series of channels, called runners that carry the molten metal into the cavity. At the end of each runner, the molten metal enters the cavity through a gate that controls the flow rate and minimizes turbulence.
Chambers called risers that fill with molten metal are often connected to the runner system. Risers provide an additional source of metal during solidification. When the casting cools, the molten metal shrinks and the additional material in the gate and risers acts to back fill into the cavities as needed. Open risers also aid in reducing shrinkage. When open risers are utilized, the first material to enter the cavity is allowed to pass completely through the cavity and enter the open riser. This strategy prevents early solidification of the molten metal and provides a source of material to compensate for shrinkage. Lastly, small channels are included running from the cavity to the exterior of the mold. These channels act as venting holes to allow gases to escape the cavity. The porosity of the sand also allows some air to escape, but additional vents are sometimes needed. The molten metal that flows through all of the channels (sprue, runners, and risers) will solidify attached to the casting and must be separated from the part after it is removed. Molten metal is poured into the mold cavity, and after it cools and solidifies, the casting is separated from the sand mold.
The accuracy of the casting is limited by the type of sand and the molding process. Sand castings made from coarse green sand impart a rough texture on the surface of the casting, making them easy to distinguish from castings made by other processes. Air-set, or no-bake, molds can produce castings with much smoother surfaces. The benefit to providing a smoother surface is discussed in more detail below but is not insignificant in improving the performance of castings made utilizing the air-set casting process. After molding, the casting is covered in a residue of oxides, silicates, and other compounds. This residue can be removed by various means, such as grinding or shot blasting. Several other surface condition benefits result from the use of the air-set process compared to the green sand process. These include benefits with regards to surface inclusions, surface porosity, laps, and scabs. Details of a comparison between required surface conditions and what can be obtained using the air-set process are provided below.
During casting, some of the components of the sand mixture are lost in the thermal casting process. Green sand can be reused after adjusting its composition to replenish the lost moisture and additives. The pattern itself can be reused indefinitely to produce new sand molds. The sand molding process has been used for many centuries to produce castings manually. Since 1950, partially-automated casting processes have been developed for production lines, some including hydraulics to compact the sand.
Green sand is an aggregate of sand (about 90%), bentonite clay or binder (about 7%), which includes pulverized coal, and water (about 3%). It is termed “green” because like a green tree branch, it contains water. The largest portion of the aggregate is always sand, which can be either silica or olivine. There are many recipes for the proportion of clay, but they all strike different balances between moldability, surface finish, and ability of the hot molten metal to degas. The coal, typically referred to in foundries as sea-coal, is present at a ratio of less than 5% and partially combusts in the presence of the molten metal leading to off-gassing of organic vapors. Also, the presence of 2-3% water results in increased occurrence of gas defects in the casting after reacting with the molten steel. Rough surface discontinuities can form as a result of the off gassing or vapors and can result in lower fatigue life for couplers and coupler parts. Given the cyclic loading to which coupler assemblies are subjected, it is important to provide as long a fatigue life as possible.
Another type of mold is a skin-dried mold. A skin-dried mold begins like a green sand mold, but additional bonding materials are added and the cavity surface is dried by a torch or heating lamp to increase mold strength. This improves the dimensional accuracy and surface finish, but lowers the collapsibility. Dry skin molds are more expensive and require more time, thus lowering the production rate.
Another type of sand that may be used in sand casting is dry sand. In a dry sand mold, sometimes called a cold box mold, the sand is mixed only with an organic binder. The mold is strengthened by baking it in an oven. The resulting mold has a high dimensional accuracy, but is expensive and results in a lower production rate.
The casting process for the manufacture of couplers has historically employed the green sand process. While this process has served the railroad industry well, there are disadvantages associated with the green sand process, such as poor material strength, porosity, and poor surface finish, resulting in shorter fatigue life, large tolerance variation, and secondary grinding/machining is often required after the casting process. Additionally, a large number of weld repairs may be required at finishing time to fix either surface or subsurface defects. Production rates are also low and include high finishing labor costs. For reasons that will become more apparent below, these disadvantages can require earlier replacement of couplers and/or knuckles, and create additional manufacturing costs that can be avoided. It would be beneficial, therefore, to use another casting process in the manufacture of railroad coupler assemblies to overcome, or at least ameliorate, these disadvantages.